In conventional lift trucks it is common practice to use an electric motor to drive a positive displacement lift pump which supplies pressurized fluid to the hydraulic load device including the lift motor. The hydraulic lift motor requires greater hydraulic flow than the other hydraulic load devices, such as the power steering system which typically requires the least hydraulic flow. Certain auxiliary load devices, such as a load handling clamp, require an intermediate value of hydraulic flow. The lift motor is energized with pressurized fluid through a manually controlled lift valve. The lift control valve is an open center, proportioning valve which provides full flow from the inlet port to the tank outlet port with no flow to the load outlet port when the valve is closed. It provides full flow from the inlet port to the load outlet port with no flow to the tank outlet port when the valve is open. In the range between the open and closed conditions there is a functional relationship between the flow to the load port and the valve movement and, even though the functional relationship may be nonlinear, the valve is called a proportioning valve. Such a proportioning valve permits feathering operation of a load device, i.e. gradual energization by gradual opening of the valve in the transition region between the open and closed conditions.
The lift motor is started by moving the lift control lever from the neutral position in the lift direction. The first increment of movement of the control lever actuates a switch which energizes the pump motor with full battery voltage to produce maximum pump flow. At first, the entire hydraulic fluid circulates from the pump outlet through the open center lift valve and returns to the tank. As the control lever is moved further, the lift valve progressively closes the tank return port and progressively opens the lift motor port so there is a decreasing flow to the tank and an increasing flow to the motor in correspondence with the movement of the lift control lever. When the flow resistance at the tank return port is sufficiently great, the back pressure causes the lift motor to lift the load. During the transition from full flow to the tank to full flow to the lift motor, energy is lost in the lift valve in the form of heat due to the pressurized fluid being reduced to the low pressure of the tank.
A general object of this invention is to provide an improved hydraulic control system which overcomes certain disadvantages of the prior art and provides for gradual energization of a load device with minimized power loss.